The present invention relates to a circuit arrangement having a sensor and a storage means for the nonvolatile storage of a charge delivered by the sensor.
Sensors are used for recording nonelectric variables and for conversion to variables which can be measured electrically, such as current, voltage or electrical charge. Nonvolatile storage of the charge delivered by the sensor has hitherto been associated with considerable complexity. As an example, CCD cameras should be mentioned in this regard, said cameras having a multiplicity of light-sensitive sensors which are arranged in a similar manner to a matrix and at which an electrical charge is generated when light is incident, The charges generated at the individual sensors are read out and converted in an amplifier unit to voltages which can be used for programming nonvolatile memory cells, such as flash EEPROMs. Besides the high level of circuit complexity, the provision of a high supply voltage suitable for programming the EEPROMs and the relatively large space requirement, which arises as a result of the provision of an amplifier circuit and the physically separate arrangement of the sensor matrix and the storage means, a particular disadvantage of such apparatuses is that the aforementioned nonvolatile memories only permit digital values to be stored.
If, by way of example, in the case of light sensors, the amount of charge generated by the sensors is dependent on brightness, part of the brightness information is inevitably lost when this charge is stored using the aforementioned storage means. If each sensor has only one associated memory cell, the stored value can be used only to distinguish whether the brightness at the sensor was above or below a prescribed threshold value More exact storage of the analog value delivered by the sensor requires analog/digital conversion of this analog value and storage of the digital value obtained in this process, the number of memory cells required for storing the digital value depending on the number of places in the digital value.
U.S. Pat. Nos. 5,325,050 and 5,332,962 describe circuit arrangements having a sensor and a storage means for the nonvolatile storage of a charge delivered by the sensor, the storage means being a capacitor having a ferroelectric storage dielectric which is at least intermittently connected to the sensor element.
The circuit arrangements described also have a DC voltage source which is intermittently connected to the ferroelectric capacitor in order to produce spontaneous polarization of the dielectric, and a polarization detector to establish whether the polarization of the ferroelectric capacitor has changed on account of a voltage signal delivered by the sensor.
In this way, as in the aforementioned circuit arrangements described, it is only possible to establish whether a prescribed maximum value for the sensor signal has been exceeded This case therefore also presents only analog/digital conversion of an analog sensor signal.
The document EP 0 402 248 A2 relates to a photosensor having an analog store connected. The way in which such a circuit arrangement can be produced easily and the means used to do so are not disclosed.
The present invention was therefore based on the object of providing a circuit arrangement having a sensor and a storage means which easily permits the storage and evaluation of analog values delivered by the sensor, which can be produced as an integrated circuit arrangement, in particular, and for which the aforementioned disadvantages do not arise.
This aim is achieved by the circuit arrangement mentioned in the introduction, in which the storage means is a capacitor having a ferroelectric storage dielectric which is at least intermittently connected in parallel with connection terminals of the sensor.
The capacitor having the ferroelectric storage dielectric permits nonvolatile storage of the charge delivered by the sensor. The capacitor is connected to the sensor, possibly via a switch. This means that it is possible to integrate the sensor and the capacitor in the same semiconductor body, the result of which is a very small space requirement for producing the circuit arrangement according to the arrangement. The matrix-like arrangement of an appropriately large number of light-sensitive sensors, such as photodiodes, which each have an associated capacitor with a ferroelectric storage dielectric, thus permits all the components necessary for detecting and storing images to be accommodated on one chip.
The present invention is not restricted to the use of light-sensitive sensors, however. Instead, it can be used anywhere where the use of sensors and storage of the values delivered by the sensors are required and where the smallest possible physical form is desirable, such as in micromechanics.
Advantageous refinements of the invention are the subject-matter of the dependent claims.
To store the charge delivered by the sensor, the sensor can be permanently connected in parallel with the connection terminals of the sensor by means of a fixed line connection. By way of example, when the circuit arrangement according to the invention is used for image detection, this is a preferred embodiment if an aperture is provided in front of the light sensors which permits the light sensors to be only intermittently exposed to light. If the light sensors are in the form of photodiodes, an approximately constant voltage drops between the connection terminals thereof upon exposure to light, the current delivered by the sensors or the charge delivered to the capacitors per unit time depending on the light intensity at the respective light sensor. The charges delivered by the individual sensors and stored at least in part in the capacitors thus represent a measure of the mean illuminance at the sensor while the aperture is open. The capacitors thus store an item of image information, the resolution of the image resulting from the number of light sensors arranged in matrix form.
In one embodiment of the invention, a switch, in particular in the form of a transistor, is connected between a terminal of the capacitor and a connection terminal of the sensor element. In this case, the switch fulfills the function of the aforementioned aperture. Turning on the switch determines the instant at which the charge generated by the sensor or sensors is to be stored in the capacitor or capacitors.
Preferably, for reading out the charge stored in the capacitor, a read and reset circuit connected to at least one terminal of the capacitor is provided.
The design and manner of operation of the circuit arrangement according to the invention are explained in more detail below with the aid of illustrative embodiments in the figures of the drawings.